The present invention relates to novel solvent-based processes for making curable epoxy resin compositions.
Articles prepared from an epoxy resin have excellent adhesion, mechanical properties, thermal properties, chemical resistance and electrical properties. These properties have led to the widespread commercial use of the articles in such items as paints, adhesives, and electrical and electronic insulation. Epoxy resin formulations used for such applications can be either a one-component system or a two-component system.
One-component systems are highly desirable for numerous reasons. For example, one-component systems allow manufacturers and consumers to avoid the complex packaging required of two-component systems, to avoid the additional mixing step of a two-component system and to avoid the possibility of an inaccurate mixing step. Further benefits of a one-component system include a reduction in variation of properties via incomplete mixing and, frequently, a longer pot life.
Amine compounds, and in particular imidazoles, are widely used as curing agents for epoxy resins because products cured with imidazoles generally exhibit highly desirable chemical and physical properties. Like most tertiary-nitrogen-containing curing agents, imidazoles react very rapidly with epoxy resin systems, even at room temperature. The resulting catalyst is generally a thermoplastic solid which has a melting point between about 70xc2x0 C. and 140xc2x0 C. The catalyst may be repeatedly melted and allowed to cool and resolidify. These catalysts are not subjected to cure or cross-linking when heated. Most conveniently, the catalysts are provided as finely divided powders, such as those capable of passing through a 200-mesh screen. Further, the resulting catalyst provides a synergistic effect when combined with an additional curing agent, such as a dicyandiamid.
The use of phenolic novolac resins as a reactant with the epoxy resin and the imidazole compound is highly advantageous. The composition resulting from the addition of the phenolic resin has a shelf life of up to five times longer than without the phenolic resin. This improvement is believed to be achieved by the formation of an acid-base complex or polysalt between the novolac phenolic resin and the product of the addition reaction between the oxirane group and the imino nitrogen. The epoxy resins which are most advantageous for the reaction with imidazole those with an epoxide equivalent weight of from about 170 to about 2000 and preferably a melting point below about 140xc2x0 C. Sufficient epoxy resin should be utilized in order to provide a ratio of oxirane groups to imidazole compound molecules between 1:1 and 2:1. Larger relative quantities of oxirane groups will result in a thermosetting or high melting reaction product which is excessively difficult to activate with heat. Smaller relative quantities of oxirane groups will result in a reaction product which melts at a temperature below 70xc2x0 C. or which contains a high proportion of residual imidazole, thus resulting in a shorter shelf life of the catalyst.
The amount of phenolic novolac resin to be included with the epoxy resin may be as much as 1.5 equivalent per molecule of the imidazole. More preferably, the ratio of phenolic novolac resin to imidazole is in the range of about 0.7-1.5 with an optimum ratio of approximately 1:1.
U.S. Pat. No. 4,066,625 discloses a unitary catalyst comprising epoxy, an imidazole and phenolic resin. The processes for the reactions between the ingredients of the catalyst, along with the various ingredients and mechanisms of reaction, are set forth in that patent and U.S. Pat. No. 4,066,625 is incorporated herein by reference as if set forth in full. No solvents are used in these disclosed processes. The solvent-free process of that patent of combining the epoxy and the imidazole produces an extremely high exotherm within minutes. Consequently, the process for manufacturing the catalyst is quite dangerous and requires extreme caution during the period in which the imidazole is added to the epoxy. Thus, it would be desirable to provide a process for manufacturing the catalyst which utilizes a solvent and eliminates the risk of the high exotherm during the addition of the imidazole to the epoxy.
The present invention provides a process for producing a catalyst without causing an extreme exotherm. The process of the present invention includes combining an amine compound curing agent with a solvent, heating the mixture, adding an epoxy/solvent mixture via slow addition, removing the solvent and then heating the remaining composition. Following the heating, a phenolic resin is added to produce the final catalyst. The final catalyst comprises an amine compound, an epoxy, phenolic resin and a residual solvent. The process may also be reversed such that the amine compound is added to the epoxy. Alternatively, epoxy resin may be added via slow addition to an initial charge solution of solvent and imidazole. Phenolic resin may be added either to the initial charge solution or via slow-addition along with the epoxy resin.
A catalyst for use with epoxy resins may be produced via a method which eliminates the exotherm upon the combination of imidazole and epoxy resin. The imidazole and epoxy components which are to be selected as the starting materials for the preparation of the catalyst are chosen for the desired properties of the resulting adduct as a catalyst. Properties which are generally important and should be considered include the catalyst""s chemical structure which promotes the curing reaction by anionic polymerization, the catalyst""s melting point, the catalyst""s compatibility with the epoxy resin which will be cured in a molten state, its quick curability and its effect of addition (high curing reactivity with the smallest amount of addition).
The starting point for the process of manufacturing the adduct is combining either an amine compound or an epoxy with a solvent. While a wide range of ratios of amine to solvent may be employed, it is most preferable to combine the amine and the solvent in a ratio of about 1:1. Preferably, the amine compound is used as the starting material. While any amine compound may be utilized for the present invention, the selection of the particular amine compound is determined by the type of epoxy compound to be combined. While it is possible to use any type of amine compound which have at least one active amino-hydrogen in their molecule with monofunctional epoxy compounds, the amine which can be combined with polyfunctional epoxy compounds is an amine compound which has only one active amino-hydrogen, i.e., having a secondary amino group, in its molecule which contributes to the addition reaction with the epoxy group. Use of compounds having at least one tertiary amino group, i.e., having no active hydrogen, is also permitted, since the presence of the tertiary amino group is desirable for increasing the concentration of the amino groups which contribute to the curing reaction of the adduct, or in other words to increase the effect of the curing agent. If this condition for combination is met, any combinations of one, two or more kinds of amine compounds may be employed.
Exemplary amines which may be utilized include imidazoles such as 2-methylimidazole; 2,4-dimethylimidazole; 2-ethyl, 4-methylimidazole; 4,5 dimethylimidazole; 2,4,5 trimethylimidazole; 2 propyl, 4,5-dimethylimidazole; 2-cyclohexyl, 4-methylimidazole; 2-butoxy, 4-allylimidazole; 2-octyl, 4-hexylimidazole; 2-ethyl, 4-phenylimidazole; 2-butyl, 5-methylimidazole; 2,5 chloro, 4-ethylimidazole; 4-methyl-2-phenylimidazole; imidazolines such as 2-methylimidazoline; piperazines such as N-methyl piperazine; anabasines such as anabisine; pyrazoles such as 3,5-dimethyl pyrazole; purines such as tetramethyl quinidine and purine; and triazoles such as 1,2,4-triazole.
Once the amine compound is chosen, a solvent is added to that amine compound. The solvent should be one which will dissolve the amine compound or the epoxy compound starting material and can precipitate the adduct in the form of particles without dissolution. Generally, a substance can dissolve in a solvent having a similar polarity. The level of polarity of a solvent is often expressed by a solubility parameter having units (cal/cm3)xc2xd. A typical range of solubility parameters of epoxy compounds will be 8 to 11 (cal/cm3)xc2xd and that of amine compounds will be 8 or greater. The solubility of the amine compound/epoxy compound adduct will be 11 to 16. Thus, in order to achieve the desired precipitation or dispersion addition reaction of the present invention, it is suitable to use a solvent having a solubility parameter of 8 to 11.
Exemplary solvents which may be utilized include 4-methyl-2-pentanone, methyl ethyl ketone; methyl isobutyl ketone; methyl isopropyl ketone; acetone; n-butyl acetate; isobutyl acetate; ethyl acetate; methyl acetate; tetrahydrofuran; 1,4-dioxane; 2-ethoxyethanol; ethylene glycol monomethyl ether; diethylene glycol dimethyl ether; methyphenylether; toluene; p-xylene; benzene; cyclohexane; methylene chloride; chloroform; trichloroethylene; chlorobenzene; and pyridine. These solvents may be utilized separately or in mixtures of two or more solvents. It is also possible to use solvents having a solubility parameter outside the range of 8 to 11 if two or more solvents are combined to bring the solubility parameter within the desired range. However, since the precise solubility parameter of the solvents to be used may naturally differ depending upon the chemical structures of the amine compound and the epoxy compound, it is essential to make a precise selection for each individual situation.
Following the mixing of the amine and solvent, the composition is heated. The composition is heated to approximately 40-150xc2x0 C. and preferably to approximately 114-118xc2x0 C. Once the composition has reached this temperature, a mixture of epoxy and solvent is added via slow addition. The solvents set forth above may be also be utilized for mixture with the epoxy. While a variety of ratios of epoxy to solvent may be utilized, it is preferred to utilize a solvent to epoxy ratio of about 1:2. Any kind of epoxy compound can be employed for producing the adduct. Exemplary epoxy compounds include monofunctional epoxy compounds such as n-butyl glycidyl ether; styrene oxide and phenylglycide ether; bifunctional epoxy compounds such as bisphenol A diblycidyl ether, bisphenol F diblycidyl ether, bisphenol S diglycidyl ether, and diglycidyl phthalate; trifunctional epoxy compounds such as triglycidyl isocyanurate, triglycidyl p-aminophenol; tetrafunctional epoxy compounds such as tetraglycidyl m-xylene diamine and tetraglycidyl diaminodiphenylmethane; and compounds having more functional groups such as cresol novolac polyglycidyl ether and phenol novolac polyglycidyl ether. The selection of epoxy compounds is also determined by the type of the amine compound to which it is to be combined. Thus, while the amine compounds having only one active hydrogen can be combined with any kind of epoxy compounds, monofunctional epoxy compounds alone can only be combined with amine compounds having two or more active hydrogens.
Following the combination of the amine compound/solvent and epoxy/solvent mixtures, the solvents are removed from the composition. The removal of the solvent may be performed by any number of processes, including the use of a trap and/or a vacuum. It is preferable to remove essentially all of the solvent, however it is likely that the composition will contain a small amount of residual solvent. Once the solvent is removed, the composition is heated to a temperature of about 120-250xc2x0 C. and most preferably about 160xc2x0 C.
The desired phenolic resin may be added to the mixture to produce the final adduct at various points in the process. The phenolic novolac resins may be added before or after the removal of the solvent or they may be added with a solvent. In an alternative procedure, the phenolic novolac resins may be added to the initial charge solution. The phenolic novolac resins which are advantageously reacted with an imidazole compound and epoxy resin to form particularly advantageous curing agents are the so-called xe2x80x9ctwo-stepxe2x80x9d resins or phenolic novolacs containing at least two phenolic groups per molecule and are usually obtained by the use of acidic catalysts by reacting phenol and formaldehyde in a mol ratio greater than 1 to 1. Commercially available examples of phenolic novolac resins are Durez 12686 (Oxychem), HRJ -2190 (Schenectady), SP-560 (Schenectady), HRJ-2606 (Schenectady), HRJ-1166 (Schenectady), HRJ-11040 (Schenectady), HRJ-2210 (Schenectady), CRJ-406 (Schenectady), HRJ-2163 (Schenectady), HRJ-10739 (Schenectady), HRJ-13172 (Schenectady), HRJ-11937 (Schenectady), HRJ-2355 (Schenectady), SP-25 (Schenectady), SP-1068 (Schenectady), CRJ-418 (Schenectady), SP-1090 (Schenectady), SP-1077 (Schenectady). SP-6701 (Schenectady), HRJ-11945 (Schenectady), SP-6700 (Schenectady), HRJ-11995 (Schenectady), SP-553 (Schenectady), HRJ-2053 (Schenectady), SP-560 (Schenectady). It is important to note that the properties of the resulting catalyst may be changed by changing the ingredients and the ratios of the ingredients.